Path Traversal in Buffalo with Api Keys

Path Traversal in a Buffalo application that uses API keys for authorization can occur when user-controlled path segments are concatenated with a base directory and the resulting path is used to serve files or access resources. If the API key is treated as part of the path or used to locate a file without strict validation, an attacker can supply sequences like ../../../etc/passwd to escape the intended directory. Because Buffalo does not automatically sanitize path segments, the developer must ensure that any user input used in file operations is normalized and restricted.

Consider an endpoint that serves documents tied to an authenticated client identified by an API key. If the API key is used as a folder name and the filename is taken directly from a request parameter, a request such as /files?api_key=CLIENT123&document=../../../etc/shadow can traverse outside the intended directory. Even though the API key scopes access to a client-specific folder, the traversal can move upward if the path is not cleaned, potentially exposing files outside that scope. This risk is higher when the API key is used naively as a directory name without validating or restricting its usage to a known base path.

Buffalo’s middleware and routing do not inherently protect against path traversal; it is the developer’s responsibility to validate and sanitize paths. When API keys are used to determine file system locations, ensure that the resolved path remains within the allowed base directory. For example, compute the absolute path using filepath.Clean and confirm that the cleaned path starts with the expected base. Failing to do so can turn an API key–scoped endpoint into a vector for reading arbitrary files if user input is not tightly constrained.

To remediate path traversal when using API keys in Buffalo, always treat API keys as identifiers for scoping, not as part of the filesystem path directly. Use a mapping from API key to a safe base directory, and ensure all file operations are anchored to that base. Never concatenate raw user input to paths derived from API keys without canonicalization and prefix checks.

Below is an example of a secure Buffalo handler that serves files for a given API key. It uses a predefined map to associate API keys with allowed base directories, cleans and validates the requested filename, and ensures the final path remains within the permitted directory.

// secure_file_handler.go
package actions

import (
    "io/fs"
    "net/http"
    "path/filepath"

    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/packr/v2"
)

// AllowedBase maps API keys to their permitted base directories.
// In production, load this from a secure configuration or database.
var AllowedBase = map[string]string{
    "CLIENT123": "/srv/app/data/client123",
    "CLIENT456": "/srv/app/data/client456",
}

// ServeFile validates the API key and filename, then serves the file safely.
func ServeFile(c buffalo.Context) error {
    apiKey := c.Param("api_key")
    requestedFile := c.Param("filename") // e.g., "report.pdf"

    base, ok := AllowedBase[apiKey]
    if !ok {
        return c.Error(http.StatusUnauthorized, errors.New("invalid API key"))
    }

    // Clean the requested filename to remove any ".." or dot segments.
    cleanFile := filepath.Clean(requestedFile)

    // Ensure the cleaned filename does not start with a path separator.
    if filepath.IsAbs(cleanFile) || cleanFile == ".." || cleanFile[:2] == ".." {
        return c.Error(http.StatusBadRequest, errors.New("invalid filename"))
    }

    // Build the full path and ensure it remains within the base directory.
    fullPath := filepath.Join(base, cleanFile)
    fullPath, err := filepath.Abs(fullPath)
    if err != nil {
        return c.Error(http.StatusInternalServerError, err)
    }

    if !strings.HasPrefix(fullPath, filepath.Clean(base)+string(filepath.Separator)) && fullPath != filepath.Clean(base) {
        return c.Error(http.StatusBadRequest, errors.New("path traversal attempt"))
    }

    // Serve the file using http.ServeFile or a similar safe method.
    http.ServeFile(c.Response(), c.Request(), fullPath)
    return nil
}

Additionally, if you use Buffalo’s static middleware to serve files, ensure that the static paths are explicitly set to directories that do not depend on API keys. For API-key–scoped files, handle downloads through a controlled action like the example above rather than exposing static directories directly. This approach prevents path traversal regardless of how the API key is presented in the request.